TP: Phase Diagrams

Phase diagrams

A pure substance cannot occupy any arbitrary point on the P,T, v space, instead being confined to a surface (i.e., any two that are identified specify the third).

For this reason, one often visualizes the phase behavior of a species by taking a projection of the PvT spaces in two dimensiones: typically in the P-v or P-T plan.

DEFINITION

A PT diagram shows the phase behavior of a single species in the pressure-temperature plane. Here, lines denote the boundaries between phases (or the points where the phases coexist).

PT diagram

DEFINITION

The saturation pressure is the coexistence pressure between liquid and vapor for a pure species and is a function of temperature (only). (see state postulate)

Note

While the vapor pressure is similar to the saturation pressure, the vapor pressure refers to the pressure in a mixture of components (i.e., the total pressure is higher).

Outcome:

Describe the difference between the saturation and vapor pressures

Pv diagram

Coexistence on a Pv diagram is shown on the liquid-vapor dome, since v is very different for liquids and vapors (up to the critical point!).

Coexistence on a Pv diagram is shown on the liquid-vapor dome.

Note

A supercritical fluid is a substance which is above its critical temperature, Tc. Here there is no vapor-liquid transition so the liquid-vapor dome ends.

definition:
  • Saturated essentially means "in phase equilibrium"
  • Superheated means at a temperature above the saturation/coexistence curve
  • Subcooled means at a temperature below the saturation/coexistence curve
  • Appendix B1, B2

On the vapor-liquid line the temperature is called the boiling point and the pressure is the vapor pressure.

On the vapor-solid line the temperature is called the sublimation point.

On the liquid-solid line the temperature is called the freezing/melting point.

Outcome:

Identify the phases present on a PT and/or Pv diagram as well as the critical point and triple point